I got a simular problem with a very persistent griefer. I had the illusion owning a popular location, mentioned in the destination guide could make a difference. It didn't.
On all attemts to get LL do something about him I got either no reply or an insane one.
During the years I learned that AR's are pretty useless. Filing AR's seems to be only having some chance of LL doing something with it if it happens massivly. Having a lot of people complaining in a short time frame, about the same issue. If you could organize that you might have a chance. With my griefer I noticed also that did have no effect.
The best suggestion I could make for you: Leave mainland. On the estates you would at least have a landlord. None of the landlords I ever worked with would allow your neighbour to stay. They also do better work in monitoring the sim, keeping lag low and keeping the neighbourhood in good standing. There are plenty of estates also offering adult rated land at pretty sharp prices.